The role of alkalizing agent on the manganese phosphating of a high strength steel part 2: The combined effect of NaOH and the amino group (NH4OH, mono-ethanolamine and NH4NO3) on the degradation stage of the phosphating mechanism

• The effect of the alkalizing agents – NaOH + NH4OH, MEA and NH4NO3 - is studied.
• These alkalizing agents do not modify the structure of the manganese phosphate coating (hureaulite).
• The NHx additives act as cathodic corrosion inhibitors during phosphating and the steel exhibits a similar Ecorr and icorr.
• The degradation of the coating is favored by the amino group additives
• These additives allow the leaching of manganese from the prismatic crystals increasing their cycles of growth.
• The effect of equilibrium, corrosion, degradation on the crystal size can be expressed through: S(t) = S0 + C(t) sin[D(t)].

This work shows the effect that presents NaOH in combination with several additives of the amino group on the formation of manganese phosphate coatings on steel. These combinations of additives does not modify the structure of the coating (hureaulite) but can affect the stages of corrosion, degradation and crystalline reorganization of the cyclic phosphating mechanism. These amino additives behave as cathodic corrosion inhibitors and are auxiliary agents in the acid leaching of manganese from the prismatic crystals of hureaulite which accelerates the crystalline reorganization stage and thus modifies the frequency of the cycles of growth and reduction of the prismatic crystals. Some of the additives act as chelating agents, retaining the Mn2 + and affecting the kinetics of precipitation and dissolution of manganese rich hureaulite crystals, but increasing the sites of nucleation and growth of the crystals (crystal density). The formation of the phosphate coating follows the cyclic phosphating mechanism for all the baths. The effects on crystal size (S(t)) of the equilibrium conditions in the bulk of the bath (S0), corrosion (C(t)) and degradation (D(t)) stages in each cycle can be resumed through the equation: S(t) = S0 + C(t) sin[D(t)].

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